Wednesday, November 16, 2011

The builders all around us

I think we have all seen many pictures like this one, of a bird feeding chicks. This goldfinch is bringing a seed to his offspring in the "standard" cup-shaped nest. As I read in Avian Architecture: How Birds Design, Engineer and Build by Peter Goodfellow, about half of all bird species make this kind of nest, from tiny hummingbirds to robins to crows. This image is from page 57 of the book; click on it for a larger version, but see the book for a full-page beauty of an image.

Such nests are about halfway along a scale of ambition, which the book seems to follow. The simplest nests are scrapes, such as the rocky rings pulled together by terns and some gulls. Some birds build small or modified cup nests inside ready-made holes, while others such as woodpeckers and burrowing owls make their own holes. Platform nests may start as a scrape, if built on the ground, but a large structure is piled up, often to raise the eggs above rainfall runoff or just the chilly soil. But the most familiar platforms are those built by raptors such as eagles, high in trees or on a cliffy ledge.

There is a spectrum from platform nests built on the ground to floating nests, with aquatic platforms somewhere in the middle: a bird will pile up a platform of rocks and mud in shallow water until it reaches the surface, then top it with plant materials for another few (or many) inches, so that it holds the eggs above the water, but is moated from predators. Other aquatic nests are woven to reeds or stalks and constructed of light materials so that they can float and support the eggs and a sitting parent. Properly attached, they can rise and fall with the water.

Birds a little more ambitious than cup-makers build a dome over a cup. At this point, five chapters have been spent on nests largely produced from plant materials, at least the top part. Chapter six is on mud nests, and this image of a cliff swallow colony is one of the most stunning in the book (page 93). Barn swallows build nests that are similar, if more open, but to my observation, are nearly always solitary. It takes hundreds of flights, carrying a small ball of mud in the beak, to build a mud nest. During the carrying, some saliva gets mixed in, which increases the adhesive properties of the mud.

We return to plant materials when we consider hanging nests, which are usually stitched or woven, such as the oriole nest. Orioles and other weaving birds work harder than most others, and it takes a long time, working with only a beak, to weave their elaborate nests. I remember getting a fallen oriole nest, and trying to figure out how the strands were put together. With my ten fingers and the help of tweezers, I was scarcely able to untwine just a strand or two.

Not all woven nests are pendulous, of course. Some look superficially like a cup nest, but are woven of stranded materials such as grass blades and hang together better than the twiggy nests with which we are most familiar. Key to their success is that they give under pressure, so the initial nest just barely fits the clutch of eggs and the sitting parent, but stretches as the chicks grow.

Mound nests may sometimes be rather simple, as the nests of these flamingos (page 109), but some are very large. Some mound-builders fill the middle of the mound with leaves and leaf litter, and bury the eggs within, so that the composting stuff keeps the eggs warm. These birds don't sit on the eggs, and typically abandon them once they are certain the composting temperature is right. Flamingos and others do incubate the eggs by sitting. Many penguin species also make mound nests.

The tenth chapter is on group nests. These cover the gamut of building styles and materials. Their common element is that multiple bird pairs nest, each in its own "apartment", in a large structure built by all the pairs.

Not all structures built by birds are for nesting and caring for young. Bowers and courts are major examples of nestlike structures that a male bird produces to impress females and induce them to mate with him. Males of bowerbirds and their relatives do not care for the young. After mating, a female goes off alone and builds a small cup nest in a hidden location.

The last chapter is half about "edible" nests, the ones used in bird-nest soup, and half about caches of food that certain woodpeckers produce, often by making a "mailbox" of holes in a dead tree trunk and filling the holes with acorns. As to the soup-nests; I can't imagine eating bird spit, which is the only construction material for the "white" swiftlet nest. Yet there is big business in cultivating swiftlets in Indonesia, where special buildings are constructed for the birds to nest in. Chacun à son gout!

The book is lavishly illustrated, and each chapter contains a "blueprint" page, with specifications for typical nests of each type, plus three or more case studies of a few species. While it is enjoyable to read right through, it is also a valuable reference book, and a good companion book to the field guides we use, which have little information on nests.